I. Field of the Invention
The present invention relates to data communications. More particularly, the present invention relates to a novel method and apparatus for extending the sequence numbering range applicable to selective repeat transmission protocols.
II. Description of the Related Art
FIG. 1 is a block diagram of a personal communication system configured in accordance with the use of the IS-95 over-the-air interface standard. The IS-95 standard, and its derivatives such as IS-95-A, IS-99 and IS-707, IS-657 and ANSI J-STD-008 etc. (referred to herein collectively as the IS-95 standards), defines an interface for implementing a digital personal communication system using code division multiple access (CDMA) signal processing techniques. Also, a personal communication system configured substantially in accordance with the use of IS-95 is described in U.S. Pat. No. 5,103,459 entitled "System and Method for Generating Signal Waveforms in a CDMA Personal communication System" assigned to the assignee of the present invention and incorporated herein by reference.
As is typical for most personal communication systems, IS-95 allows mobile telephone service to be provided to a set of wireless terminals (typically cellular telephones) using a set of base station 12 coupled to the public switched telephone network (PSTN) 18 by a transmitter controller (BSC) 14 and a mobile switching center (MSC) 16. During a telephone call, a wireless terminal 10 interfaces with one or more base stations 12 using CDMA modulated radio frequency (RF) signals. The RF signal transmitted from the base station 12 to the wireless terminal 10 is referred to as the forward link, and the RF signal transmitted from the wireless terminal 10 to the base station 12 is referred to as the reverse link.
Under the IS-99 and IS-707 standards (referred to hereinafter simply as IS-707), an IS-95 compliant communications system can also provide data communications services. Data communications services allow digital data to be exchanged using receiver 10 and the RF interface to one or more transmitters 12. Examples of the type of digital data typically transmitted using the IS-707 standard include computer files and electronic mail.
In accordance with both the IS-95 and IS-707 standards, the data exchanged between a wireless terminal 10 and a base station 12 is processed in frames. To increase the likelihood that a frame will be successfully transmitted during a data transmission, IS-707 employs a radio link protocol (RLP) to track the frames transmitted successfully, and to perform frame retransmission when a frame is not transmitted successfully. Retransmission is performed up to three (3) times IS-707, and it is the responsibility of the higher layer protocols to take additional steps to ensure the frame transmitted is successfully.
In order to track which frames have been transmitted successfully, IS-707 calls for an eight-bit sequence number to be included in each frame transmitted. The sequence number is incremented for each frame from 0 to 256 and then reset back to zero. An unsuccessfully transmitted frame is detected when a frame with an out of order sequence number is received, or an error is detected using CRC checksum information or other error detection methods. Once an unsuccessfully transmitted frame is detected, the receive transmits a negative-acknowledgment message (NAK) to the transmit system that includes the sequence number of the frame that was not received. The transmit system then retransmits the frame including the sequence number as originally transmitted. If the retransmitted frame is not received successfully, a second negative-acknowledgment message is sent to the transmit system. The transmit system typically responds by notifying the controlling application or network layer of the failed transmission.
Under IS-95A and IS-707, frames are transmitted once every 20 milliseconds (ms). Thus, an eight-bit sequence number can track 256 frames transmitted over a five (5) second interval. Five seconds is typically sufficient to allow a failed frame transmission to be detected, and a retransmission to be performed, and therefore an eight-bit sequence number provides sufficient time for frame retransmission. Thus, retransmitted frames can be uniquely identified without ambiguity caused by a sequence "wrap-around" whereby the eight bit sequence number repeats.
Since the original development of IS-95A and IS-707, however, additional protocols and standards have been proposed and developed that allow data to be transmitted at greater rates. Typically, these new protocols and standards use the same frame structure as IS95A and IS707 in order to maintain as much compatibility as possible with pre-existing systems and standards. However, while maintaining compatibility with pre-existing standards and systems is desirable, the use of the same type of frame within these higher rate protocols and standards substantially increases the number of frames that are transmitted during a given period of time. For example, if the transmission rate is increased by a factor of four, the time required to transmit 256 frames is reduced to 1.25 seconds, rather than the five seconds required previously. A time period of 1.25 seconds is typically insufficient to allow a failed frame transmission to be detected, and a retransmission attempted, before the eight-bit sequence number repeats. Thus, the use of an eight-bit sequence number is insufficient to allow unique identification frames for the time period necessary to perform the desired retransmission sequence.
While the number of bits in the sequence number could be increased, such an increase would substantially alter the frame format and therefore violate the goal of maintaining substantial compatibility with previously existing systems and standards. Thus, the present invention is directed to a method and apparatus for extending the sequence number range without modifying the number of bits used for the sequence number.